I dont really have any original advice.. LOL (sorry)
I have just looked at the most popular.. and why?
price is ALWAYS going to be a popular 'reason' behind a customs purchase.. (always)..
by size seems most popular and gives the end user a way to measure their needs vs cost.. as well as protects your interests (such as resources/cost of doing business).. where the break even/profit margin is for you there.. is up to you guys to work out. :)
I would say 'price' and the way users can interface/communicate/send files to you guys (easy, stream-lined..etc) will be KEY to your success..
I mean I have ZERO interaction with iTeadStudios (unless a problem occurs of course).. its ALL automated..
I reply to email with my order# and my gerber files to be created.. I wait a few days.. I get automated acceptance email.. and life is good.. and then wait for package to arrive..
easy for me..
no hand holding or time wasted on customer 'sales' (only customer service) for them..
Im not sure how it is over there.. of if many share my opinion.. but or ME..
the 'hacker/makerspace' movement is getting bigger and bigger..
its on the rise and only getting more attention..
its a very nice market to target...
even if yo prices were a "LITTLE" more than SEED/iTeadStudios.. I think you would still do fairly well if quality was same/better..
I believe they benefit from repeat/quantity of customers..